Darlington Borough Council Level 2 Strategic Flood Risk Assessment

Final Report

October 2010

Karen Johnson Spatial Planning Darlington Borough Council Town Hall Feethams Darlington DL1 5QT

JBA Office

JBA Consulting The Brew House Wilderspool Park Greenall's Avenue Warrington WA4 6HL JBA Project Manager

Sam Wingfield Revision History

Revision Ref / Date Issued Amendments Issued to Karen Johnson and Caroline Issues Paper 1.0. April 2010 Brumwell Updated from EA guidance Karen Johnson and Caroline Draft version 2.0 and DBC comments on issues Brumwell paper. Updated after comments from Karen Johnson and Caroline Final version 3.0 EA and DBC. Brumwell

Contract

This report describes work commissioned by Karen Johnson, on behalf of Darlington Borough Council, by an email dated 04/02/10. Darlington Borough Council’s representative for the contract was Karen Johnson. Sam Wingfield of JBA Consulting carried out this work.

Prepared by ...... Sam Wingfield

Reviewed by ...... Jonathan Cooper and Chris Isherwood

Purpose

This document has been prepared as a Final Report for Darlington Borough Council. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. JBA Consulting has no liability regarding the use of this report except to Darlington Borough Council.

2010s4000 DBC Level 2 SFRA V3.0.doc i

Acknowledgments

JBA would like to thank Karen Johnson who assisted in the production of this report. Copyright © Jeremy Benn Associates Limited 2010 Carbon Footprint

346g

A printed copy of the main text in this document will result in a carbon footprint of 272g if 100% post-consumer recycled paper is used and 346g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is a carbon neutral company and the carbon emissions from our activities are offset.

2010s4000 DBC Level 2 SFRA V3.0.doc ii

Contents

1. Introduction ...... 1 1.1 Background ...... 1 1.2 Scope and Objectives ...... 1 1.3 Study Area ...... 3 1.4 Outline Methodology ...... 5 2. Flood Risk and Flood Defence Review ...... 7 2.1 The River Skerne ...... 7 2.2 Cocker Beck ...... 12 3. Town Centre Fringe Assessment ...... 15 3.1 Introduction ...... 15 3.2 Development Implications and Issues for Existing Risk ...... 18 3.3 Mitigation Strategy ...... 34 3.4 Next Stages ...... 38 3.5 Summary ...... 40 4. Critical Drainage Areas ...... 42 4.1 Introduction ...... 42 4.2 Detailed Surface Water Mapping ...... 43 4.3 Critical Drainage Areas ...... 43 4.4 Green Infrastructure Opportunities ...... 48 4.5 Future Studies ...... 49 4.6 Surface Water Drainage and Development ...... 50 5. Conclusions and Next Stages ...... 52 5.1 Town Centre Fringe ...... 52 5.2 Surface Water Management ...... 53 Appendices...... I A. Figures ...... I B. Glossary of Terms ...... II

2010s4000 DBC Level 2 SFRA V3.0.doc iii

List of Figures

Figure 1 – Indicative Town Centre Fringe Residential Sites ...... 2 Figure 2 - The candidate critical drainage areas ...... 4 Figure 3 – NFCDD raised defence locations to the north ...... 8 Figure 4 – Looking u/s from John Street at reference 723 ...... 9 Figure 5 – Looking u/s from John Street at reference 725 and 726 ...... 9 Figure 6 – Looking d/s from John Street at reference 722 and 721 beyond that ..... 10 Figure 7 – Looking u/s from Chestnut Street at reference 719 ...... 10 Figure 8 - NFCDD raised defence locations to the south ...... 11 Figure 9– looking d/s from Priestgate at reference 715 ...... 11 Figure 10 - Looking downstream at Cocker Beck from Northgate ...... 12 Figure 11 – Current Environment Agency Flood Zones (version 3.15) ...... 16 Figure 12 – Draft revised Flood Zones ...... 17 Figure 13 – a) Main flood flow pathway early stages ...... 18 Figure 13 – b) Main flood flow pathway later stages ...... 18 Figure 14 – Existing Risk Flood Extents ...... 20 Figure 15 – Existing Risk Flood Depths ...... 22 Figure 16 – Existing Risk Flood Depths ...... 23 Figure 17 – Comparison of the raised TCF sites (or flood defences) against the existing risk for the 1 in 100 year+cc event ...... 26 Figure 18 – Comparison of the 30m floodplain corridor against the existing risk for the 1 in 100 year+cc event ...... 30 Figure 19 – The 1 in 100 year+cc existing risk event (blue) and the increase in extent from the corridor option (red) ...... 31 Figure 20 – Comparison of raising TCF site B against the existing risk for the 1 in 100 year+cc event ...... 33 Figure 21 – Potential bund location to remove short term flood risk to Site B ...... 34 Figure 22– Detailed surface water mapping in Pierremont ...... 44 Figure 23– Detailed surface water mapping in the centre of Darlington ...... 46 Figure 24– Detailed surface water mapping at Eastbourne ...... 47 List of Tables

Table 1 - Suggested Screening Criteria for Mitigation Measures ...... 21 Table 2 - Flood Hazard Thresholds ...... 24 Table 3 – Compensatory flood storage volumes...... 28 Table 4 – Mitigation strategy for the TCF sites ...... 36 Table 5 - Potential GI Opportunities in Local Plan (LP) Allocations’ ...... 49 Table 6 - Suitability of SUDS Techniques ...... 51 Table 7 - Strategic SUDS Applicability ...... 51

2010s4000 DBC Level 2 SFRA V3.0.doc iv

2010s4000 DBC Level 2 SFRA V3.0.doc v

1. Introduction

This Level 2 SFRA follows on from Volumes I, II and III which formed the Level 1 SFRA. The purpose of this report is to provide an assessment of flood risk at one of the key regeneration areas in Darlington, where there is a risk of flooding. These sites are referred to as the Town Centre Fringe Sites and are comprised of residential and employment site allocations. These sites were identified in the Level 1 SFRA as being at risk of flooding but also necessary for Darlington BC's regeneration and development plans.

1.1 Background JBA Consulting was commissioned in February 2010 by Darlington Borough Council (Darlington BC) to undertake a Level 2 Strategic Flood Risk Assessment (SFRA). The Level 1 SFRA was submitted in December 2009. This Level 2 SFRA provides more detail for the Town Centre Fringe sites and the Critical Drainage Areas (CDAs) and follows on from the Level 1 reports. The Level 1 and 2 SFRAs for Darlington BC have been prepared in accordance with current best practice, Planning Policy Statement 25 Development and Flood Risk (PPS25) and the PPS25 Practice Guide (December 2009). This document comprises the Level 2 assessment. After ongoing consultation, the outcomes of this SFRA have been agreed, in principle, with the Environment Agency.

1.2 Scope and Objectives This study will provide an assessment of flood risk at the Darlington Town Centre Fringe (TCF) area (see Figure 1), where regeneration conflicts with flood risk. This flood risk information will inform the Local Development Framework (LDF) and the policies and proposals produced for the development. This study will also assess the candidate Critical Drainage Areas (cCDAs) and confirm if they are a CDA and should be taken forward into a Surface Water Management Plan (SWMP). The Darlington BC Level 1 SFRA (Volume II) has provided sufficient data and information to inform the application of the Sequential Test. This information was based on current available information, including: • Flood Zone maps • Modelled flood outlines • Flood risk management measures maps • Surface water flooding maps • Climate change maps

However, a large proportion of the TCF sites are shown to be within Flood Zone 2 due to flooding from the River Skerne. These sites could not be moved to areas of lower flood risk (Sequential Test) as they are key regeneration sites and required to help deliver approximately 650 dwellings and 17ha of employment land (within the Town Centre) by 2026 (Core Strategy: Publication Draft1), a more detailed assessment is therefore required. This was to be the basis of the Level 2 SFRA but after a review of the River Skerne hydraulic model, it appears that the TCF sites are at risk from the 1 in 100 year flood (Flood Zone 3) as well. As a result, the Environment Agency has commissioned JBA

1 Darlington Local Development Framework Core Strategy: Publication Draft, Darlington Borough Council, August 2010-10-25

2010s4000 DBC Level 2 SFRA V3.0.doc 1

Consulting to update the Flood Maps on the River Skerne concurrently with this Level 2 SFRA.

Figure 1 – Indicative Town Centre Fringe Residential Sites

OS Licence: © Crown Copyright 100023297 (2010).

This also means that is a greater risk of flooding at the TCF and the Exception Test will need to be passed if these sites are to be approved during examination of the Core Strategy. This will involve a more thorough study in order to truly understand the mechanisms of flood risk around these key regeneration sites. A key objective of the Level 2 SFRA is therefore to inform the application of the Sequential Test, by assessing the TCF development sites at medium and high flood risk, and assist Darlington BC in establishing whether the requirements of the Exception Test can be met as outlined below:

2010s4000 DBC Level 2 SFRA V3.0.doc 2

a. It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the LDD has reached the ‘submission’ stage (see Figure 4.1 of PPS12: Local Development Frameworks) the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal (SA); b. The development must be on developable previously-developed land or, if it is not on previously-developed land, that there are no reasonable alternative sites on developable previously-developed land; and c. A site-specific Flood Risk Assessment must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.

Whilst the Exception Test process makes it possible to identify areas where developments can be built safely, it must not been seen as an opportunity to place inappropriate development in flood risk areas. It is a useful planning tool that can justify the acceptability of the residual risks remaining after the mitigation measures have been applied. In order to establish whether applying the Exception Test is justified or can then be satisfied, namely part c), the Level 2 SFRA considers the detailed nature of the flood hazard, taking account of the presence of flood risk management measures such as flood defences. The detail nature of the flood hazard within a flood zones includes: • Flood probability; • Flood depth; • Flood Velocity; and • Rate of onset of flooding.

These factors can be significantly affected by the presence of flood defences or any other infrastructure which acts as a flood defence. Flooding behind such infrastructure can occur either as a result of: • Constructional or operation failure of the defence, either in whole or in part (breach); or • Water levels rising to exceed the level of the defence (overtopping); or • Overloading of the surface water drainage system, either due to its own limited capacity, or being unable to discharge due to high water levels outside the defended area.

By facilitating the application of the Exception Test, the Level 2 SFRA technical work will also provide supporting evidence to the possible mitigation measures that would enable the development to proceed.

1.3 Study Area This study will focus on flood risk from the river Skerne to the TCF sites between Victoria Road (south of the police station) and the railway line (to the south of Albert Road). This area can be seen in Figure 1. There is currently a mixture of residential and employment land use proposed here, but the area will be assessed independently of this in order to identify where the least vulnerable land use should be located. However, there will also need to be an allowance for other planning factors i.e. residential use may not be appropriate in the lowest flood risk zones. The TCF regeneration area is separated by the River Skerne with development proposed on either side. If the River Skerne overtopped its banks, parts of the TCF area would be

2010s4000 DBC Level 2 SFRA V3.0.doc 3

flooded. In addition, Cocker Beck converges with the Skerne south of John Street. This also presents a flood risk to the TCF sites and there is a combined probability risk of both watercourses flooding at the same time. The Level 2 SFRA will also look at the candidate Critical drainage Areas (cCDAs - see Figure 2) and confirm if they should be brought forward as CDAs to be studied in a Surface Water Management Plan (SWMP). The Level 1 SFRA identified the following areas as cCDAs: • Eastbourne • Pierremont • Darlington town centre east of the River Skerne

This study area will cover these areas in addition to any green infrastructure opportunities and surface water flooding areas identified by the more detailed surface water mapping.

Figure 2 - The candidate critical drainage areas

OS Licence: © Crown Copyright 100023297 (2010).

1.3.1 Current Land Use and Proposed Development Currently, much of the TCF area between Chesnut Street and the railway line to the east of the Skerne is composed of warehouses with yards for the storage of goods. Compared to the town centre, this part of the urban area is not intensively developed and while the regeneration opportunities here are clear, redeveloping this area with higher density, more intensive forms of development would reduce the amount of floodplain in the area shown to be at risk of flooding. If floodplain is reduced, compensatory flood storage would be required. This will be discussed in more detail later in the report.

2010s4000 DBC Level 2 SFRA V3.0.doc 4

To the west of the River Skerne, development is denser with buildings built to the bank of the watercourse. However, there are some less developed areas to the east bank of the Skerne (e.g. North of John Street and North of Russell Street). Throughout the TCF area, the land use is for employment rather than residential, so there would be a change in land use vulnerability2 if regeneration which includes housing goes ahead. There are also highly vulnerable uses2 in the regeneration area (fire and ambulance station) which may be replaced by less vulnerable uses. The TCF is a key regeneration area in Darlington. Mixed use including residential development is the preferred option for this area in the Core Strategy. Draft policies for the TCF propose 650 dwellings on previously developed land in this area as key to sustaining the Core Strategy’s housing strategy. The Council is preparing a masterplan for the wider area to be followed by a more detailed Area Action Plan (AAP) for part of the area. This Level 2 SFRA will inform the masterplan and subsequently the AAP by identifying the locations of greatest flood risk including the main flood pathways and influencing development around this risk and by proposing a mitigation strategy.

1.4 Outline Methodology As previously mentioned, the Level 1 SFRA was based on currently available information such as the Environment Agency Flood Map and data collected from key stakeholders during the consultation process. To ensure the scale and nature of the risks are truly understood, the Level 2 SFRA is based on new detailed modelling information. Flood outlines are normally derived from 1D hydraulic models simply by extrapolating a predicted water level in the channel across the floodplain using topographical data to estimate those areas that will be inundated. Whilst this method is suitable to create the Environment Agency Flood Maps, as used within the Level 1 SFRA, this approach is not always appropriate in instances where there are likely to be complex flow patterns and where water levels in the floodplain may differ significantly from those in the watercourse. In addition, as mentioned previously, Flood Zone 3 on the River Skerne needs to be revised as it currently shows that the 1 in 100 year event stays in bank. After reviewing the hydraulic model, the 1 in 100 year flood would overtop in and around the TCF. The new modelling will therefore upgrade the existing 1D hydraulic model of the Skerne to a 2D model. The 2D modelling approach is seen as crucial in urban areas, in order to simulate the conveyance of flood waters across the floodplain and to gain a more detailed understanding of the flood risk. This improved modelling will also allow a revision of the Flood Zones on the Skerne. A 1D hydraulic model has recently been completed for Cocker Beck (which also poses a risk to some of the TCF sites). This model will be converted and integrated into the 2D model of the Skerne. This will allow an assessment of the risk if Cocker Beck and the River Skerne were in flood. The new modelling includes: • Converting the existing 1D ISIS model of the Skerne into a 2D ISIS TUFLOW model. • Converting the existing 1D HECRAS model of Cocker Beck into ISIS and combining it with the 2D ISIS TUFLOW model of the Skerne. The modelling includes the following scenarios: • Raising the proposed TCF development area above the 1 in 100 year+ climate change flood event to find out the impact upstream and downstream. • Only raising Site B. • Providing raised flood defences along the Skerne, to prevent flooding of the TCF sites.

2 Table D.2: Flood Risk Vulnerability Classification, Planning Policy Statement 25: Development and Flood Risk, Communities and Local Government, March 2010

2010s4000 DBC Level 2 SFRA V3.0.doc 5

• Strategic upstream flood storage assessment. • A 30m wide amenity corridor along the River Skerne to increase flood conveyance. The following flood return periods were modelled for the above scenarios: • 1 in 25 year fluvial event • 1 in 100 year fluvial event • 1 in 100 year fluvial event with climate change • 1 in 1000 year fluvial event

The results of the modelling and the mapping outputs are discussed in Chapter 4 of this report. This includes a summary of the modelling completed, the results and how this impacts on the potential development allocations (i.e. can they be developed safely without increasing risk to others and what mitigation measures would be required to make them so). Within this report, different flood events are referred to, for example the 1 in 5 year event. A 1 in 5 year event is one that has a 20% chance of happening in any year (this is a relatively minor flood). A 1 in 100 year flood has only a 1% chance of happening in any year, but its effects can be enormous. Where climate change flood events have been modelled, this is referred to as the 1 in 100 year event +cc. The climate change scenarios have been modelled by increasing fluvial flows by 20%.

2010s4000 DBC Level 2 SFRA V3.0.doc 6

2. Flood Risk and Flood Defence Review

2.1 The River Skerne 2.1.1 Previous Studies Section 2.3 in Volume II of the DBC Level 1 SFRA provides a review of the River Skerne in Darlington using all the studies that have been completed and includes a review of flood history and general flood risk in Darlington. The most recent study on the River Skerne in Darlington is the River Skerne Pre Feasibility Study3. This should provide the most up to date information on the flood defences and flood risk. This study stated that for the 1 in 100 year event, there is no flooding of properties within Darlington town centre (John Street to Victoria Embankment), but said that through this reach, the hinterland is lower than the river bank and the peak water levels. It is believed that the height of the river banks prevent flooding to a large part of central Darlington. The prefeasibility study also states that the standard of protection offered by the bank heights through much of Darlington town centre is in excess of the 1 in 200 year event. This appears to contradict with the modelling completed for this Level 2 SFRA. This modelling shows that the 1 in 100 year flood level is above the bank tops. In addition, a site visit seemed to show that there are very few raised flood defences on the Skerne through Darlington (see Section 2.2.3). The full results of the modelling are described in Chapter 4. There is a long history of flooding on the Skerne. The Skerne has flooded at least 20 times since the 1850s and the centre of Darlington has been affected during many of these events. This includes the flooding of properties along Valley Street, John Street, Oxford Street, Mount Street and Parkgate in 1967 (the town centre fringe area). Most recently the Skerne has flooded in 2000 and 2001. The Level 1 SFRA provides more details on the flood history of the Skerne. The assumption in the Prefeasibility study is that the works completed in the past have removed flood risk up to at least the 1 in 200 year event. This is based on the March 1979 event when a flood flow not seen since 1875 passed through the Skerne. Preistgate bridge was overtopped but no properties were flooded. However, a review of the Prefeasibility model and the 2D modelling update appears to show that the probability of flooding is much higher. 2.1.2 NFCDD Under the Defra High Level Targets, flood and coastal defence operating authorities are required to develop the National Flood and Coastal Defence Database (NFCDD). The Environment Agency are leading the development of the database, in partnership with local authorities. This database records the location of all the flood defences in the UK. The database contains information on the type of defence, composition, height, length and the standard of protection it offers. However, the amount and accuracy of this information varies from place to place. It has been recognised by the Environment Agency that the NCFDD output along the Skerne in Darlington needs updating and may be inaccurate in places. A site visit was undertaken for this Level 2 SFRA which confirms that NFCDD needs updating here. Some of the differences are summarised in section 2.3.2 below. 2.1.3 Site Visit A site visit was undertaken to see if the raised defences recorded in NFCDD and identified in the Pre Feasibility model and report are accurate. This will allow an 'existing risk'

3 Environment Agency, North East Region, National Engineering And, Environmental Consultation Agreement, River Skerne, Pre-Feasibility Study Report, 19th September 2003

2010s4000 DBC Level 2 SFRA V3.0.doc 7

scenario to be modelled for this Level 2 SFRA. This will include all raised defences and any structure that would prevent flooding during the 1 in 100 year event upwards. The current NFCDD output shows that there are raised defences along the River Skerne through Darlington. NFCDD states that some of these defences are private, some are owned by the Environment Agency and some are non flood defences structures but raised at the river bank. Figures 3 and 8 show the raised defences from NFCDD in purple. The arrow points the NFCDD reference number to the flood defence section. The photos in Figures 4 to 7 and 9 show the different ‘defence’ sections that these reference numbers refer to.

Figure 3 – NFCDD raised defence locations to the north

726

725 723

722

721

719

OS Licence: © Crown Copyright 100023297 (2010)

At reference 723 (see Figure 3), NFCDD states that there is a private, non flood defence structure here. Figure 4 shows a fence attached to a low wall. Further upstream there is no longer a wall and the bank height is the ground level. As this is not a flood defence structure and the low wall gives way to ground level allowing any flood water to outflank the raised wall, this will not be included in the existing risk (defended) runs.

2010s4000 DBC Level 2 SFRA V3.0.doc 8

Figure 4 – Looking u/s from John Street at reference 723

At reference 725 and 726, NFCDD states there is a stone and brick retaining floodwall which is a private flood defence structure. Figure 5 confirms the presence of this structure which is approximately 1m wide and 1.5m high. The condition of this retaining wall is not known, however, from observation alone it appears that the wall is wide enough not to breach during the 1 in 100 year event. This wall will therefore be included in the existing risk model runs.

Figure 5 – Looking u/s from John Street at reference 725 and 726

At reference number 722, NFCDD states that there is a concrete flood defence wall which is privately owned. Although this wall is not as wide as the upstream wall, from observation (see Figure 6), it is expected that it would remain as a flood defence during the 1 in 100 year event. This wall will therefore be included in the existing risk model runs.

2010s4000 DBC Level 2 SFRA V3.0.doc 9

Further downstream at reference 721 NFCDD states that the building frontages act as a defence to flooding. However, these are not official flood defence structures (just buildings) and flood water could get into the buildings from the side. The existing risk model runs will therefore not model the buildings here as a raised defence.

Figure 6 – Looking d/s from John Street at reference 722 and 721 beyond that

At reference 719, NFCDD states that there is an Environment Agency owned concrete flood wall. However, during the site visit, no flood wall could be seen. Figure 7 shows a stone wall, which ties in with the ground level and a metal fence. The existing risk model runs will therefore not include a flood defence structure here.

Figure 7 – Looking u/s from Chestnut Street at reference 719

2010s4000 DBC Level 2 SFRA V3.0.doc 10

Figure 8 - NFCDD raised defence locations to the south

715

OS Licence: © Crown Copyright 100023297 (2010)

Figure 9– looking d/s from Priestgate at reference 715

2010s4000 DBC Level 2 SFRA V3.0.doc 11

The final NFCDD flood defence location is shown in the right bank of the Skerne looking downstream from Priestgate (see Figure 8). NFCDD states that there is a private flood wall here. From Figure 9, it can be seen that the stone wall ties in to the ground level. A flood defence line will therefore not be modelled here.

2.2 Cocker Beck 2.2.1 Introduction Section 2.4 in Volume II of the DBC Level 1 SFRA provides a review of Cocker Beck and includes a review of flood history and general flood risk from Cocker Beck. Figure 10 shows Cocker Beck before its confluence with the RIver Skerne at Northgate. The information on Cocker Beck gathered during the Level 1 SFRA includes the following: • By 1966, a flood alleviation scheme designed to improve the capacity of the River Skerne had been completed. The works included diverting the majority of Cocker Beck's flow into the River Tees via the Baydale Beck. • There are records of flood problems along West Beck and Cocker Beck dating back to 1975. Flooding of fields at the Cocker Beck confluence was also recorded in October 1976. • After an internet search, it was shown that Cocker Beck has recently been desilted by the Environment Agency, adjacent to Westbrook Terrace. It had been noted that the silt and debris which had built up in the beck prevents the drains from discharging effectively during heavy rain, which may lead to flooding on North Road.

Figure 10 - Looking downstream at Cocker Beck from Northgate

2.2.2 Background Until very recently, there has not been a detailed study on Cocker Beck. In 2009, a flood risk mapping study was completed by the Environment Agency for Cocker Beck4. This provides detailed modelling and flood risk information for this watercourse. The new flood

4 Environment Agency, Cocker Beck, Flood Risk Mapping Study, March 2009, Final Report

2010s4000 DBC Level 2 SFRA V3.0.doc 12

extents produced during the Cocker Beck flood mapping study will replace the existing Flood Zone maps which are shown in the Level 1 SFRA (although it is not known when this will be). The (HEC-RAS) Cocker Beck model produced for the flood mapping study has been combined with the 2D hydraulic model of the Skerne to give a combined assessment of flood risk from both of these watercourses (see Section 1.4). The main flood risk information from the Cocker Beck flood mapping study is described below. Cocker Beck is a small tributary of the River Skerne, with a natural catchment area of 42 km2. However, runoff from the majority of this area is now diverted directly into the Tees via Baydale Beck, alleviating a flooding problem in the lower, urban part of the catchment. The remaining area which drains through Darlington town centre has a catchment area of 11 km2. The river and catchment gradients are gentle on Cocker Beck. Steep gradients are normally a factor that contributes to flood risk. However, the soils in the Cocker Beck catchment are a uniform mix of loam and clay, which becomes seasonally waterlogged. Rapid runoff from this saturated soil is made worse by the heavy urbanisation of the catchment. The lower Cocker Beck catchment leading up to the confluence with the Skerne is urban although the watercourse passes through green open spaces in many locations. As Cocker Beck approaches the confluence with the Skerne, it becomes an urbanised watercourse and is confined at both sides by buildings (see Figure 10). The channel is believed to still follow its natural course here, but in places has been engineered and lined. Most of the urbanisation in the Cocker catchment is longstanding residential development, but there is a significant area of new development at West Park, on the former Darlington Chemical and Insulating Works site in Faverdale. The current Flood Zone map shows that large parts of Cockerton at risk of flooding from 1 in 100 year event. However, these maps are based on the natural catchment without the Baydale Beck diversion. There are records of flooding in Cockerton, particularly at Newton Lane, caused by West Beck, since the 19th Century. There are no formal flood defences on the Cocker Beck below the Baydale Beck diversion point. Darlington Borough Council propose to return a small flow to the presently dry reach of Cocker Beck between the diverting embankment and the West Beck confluence. A previous Flood Risk Assessment for the Council found that this may cause increased risk downstream, particularly where a culvert at Waterside becomes surcharged. There is a history of flooding from Cocker Beck although not as significant as from the Skerne. However, there has not been significant flooding from Cocker Beck since the flow was diverted to Baydale Beck in 1966. This appears to show that these works have reduced the majority of flood risk from Cocker Beck. The next section describes flood risk following the 2009 flood risk mapping. 2.2.3 Flood Risk following Flood Risk Mapping The new flood risk mapping shows a significant reduction in flood extent compared to the current Flood Zone map (see Figure 12 and Figure A1 in Appendix A for the new flood mapping extents). The greatest reduction in Flood Zone area was in the Cockerton area of Darlington, where a large part of the existing Flood Zone 3 which contained dense housing has been removed. The main reason for the differences between the existing and new Flood Zones is due to including the diverting embankment in the hydraulic modelling, so that most flow goes down Baydale Beck. Using a more accurate digital elevation model (DEM), and using a better representation of the watercourse capacity compared to the Flood Zone methodology, are additional reasons for the differences.

2010s4000 DBC Level 2 SFRA V3.0.doc 13

At the confluence with the Skerne (the area where the Town Centre Fringe sites are), the flood extent is also reduced. However, flood risk form the River Skerne could still occur here. Darlington BC, in partnership with the Environment Agency, propose to return a limited amount of flow to the reach of Cocker Beck between the diverting embankment and the West Beck/Cocker Beck confluence to improve the amenity value of the area. The modelling has shown that these proposed works which includes an improved culvert will not increase flood risk downstream.

2010s4000 DBC Level 2 SFRA V3.0.doc 14

3. Town Centre Fringe Assessment

3.1 Introduction Darlington Borough Council (DBC) are currently developing their Core Strategy as part of their Local Development Framework (LDF). Before being adopted, the Core Strategy will undergo independent examination. In order to meet the required housing numbers identified in the Core Strategy, Darlington BC has identified a number of strategic locations for residential development and/or employment development. The key strategic brownfield, regeneration location for housing within the urban area is the Town Centre Fringe (TCF), which is made up of a number of potential sites (see Figure 1). Across the wider area the LDF Core Strategy: Publication Draft (August 2010). The TCF is seen as a key area for the regeneration of Darlington and for meeting housing numbers. The TCF development area is broken down into different land parcels and labelled A to H. These development sites will come forward at different times. It is anticipated that sites A, B and H are likely to come forward within the next 5 years. There are more constrains upon sites E and F, so it will take longer for these sites to come forward. This is taken into account when assessing these sites, as any mitigation strategies may need to be implemented in a sequence. The Level 1 SFRA assessed Darlington BC’s proposed development sites, including the Town Centre Fringe (TCF). Since the Level 1 SFRA, site D has been removed, following the approval of planning permission. An assessment of the existing, available information showed that parts of the TCF are within Flood Zone 2 (medium risk of fluvial flooding from the River Skerne) and a small part is in Flood Zone 3 (high risk of flooding from Cocker Beck). However, Flood Zone 3 for the River Skerne does not extend beyond the river banks. Figure 11 below shows the current Environment Agency Flood Zones and the TCF sites. A Level 2 SFRA was proposed for the TCF sites as there was a large Flood Zone 2 coverage. The intention was for the Level 2 SFRA to assess how the overall regeneration area could manage this risk. However, after reviewing the 1D hydraulic model, constructed in 2003, that was used to produce the Flood Zones used for the Level 1 SFRA, it was concluded that the model needed to be updated in a number of areas. This is because the existing model had copied cross sections for a number of different river reaches. As a result, the model does not show the 1 in 100 year flood overtopping the Skerne, when in fact it should. In addition, the model used to produce the current Flood Zones is a defended model. The Flood Zones should be undefended. Many of the river bank levels were therefore shown to be too high (incorrectly preventing flooding). It was decided, following consultation with the Environment Agency, that an integrated river floodplain 2D model would be constructed to correct these issues. 2D modelling allows flood pathways to be modelled away from the river, whereas 1D models show a flood extent by joining up the river cross sections. This new model has been constructed to update the flood extents and to produce flood depth and hazard outputs.

2010s4000 DBC Level 2 SFRA V3.0.doc 15

Figure 11 – Current Environment Agency Flood Zones (version 3.15)

Legend

Development Allocations Flood Zone 3 V 3.15 Flood Zone 2 V 3.15

OS Licence: © Crown Copyright 100023297 (2010)

After these necessary model updates were made, the 1 in 100 year flood event on the Skerne (Flood Zone 3) is shown to overtop and put many parts of the TCF sites at a high risk of flooding. However, Cocker Beck shows a reduced flood extent. These proposed Flood Zone updates can be seen in Figure 12. According to PPS25, this changes the situation significantly. For residential development to be approved in Flood Zone 3, the Exception Test needs to be completed (following the Sequential Test). The Exception Test needs to show that the development will be safe once constructed and it will not increase flood risk elsewhere. In order to do this, detailed hydraulic modelling is required which will provide flood extent, depth, velocity and hazard outputs. In addition, more complex mitigation measures are often required. The Level 2 SFRA has been completed to indicate whether the TCF sites will pass the Exception Test

2010s4000 DBC Level 2 SFRA V3.0.doc 16

and what mitigation measures will be required in order for residential development to be approved. If the TCF sites were only at risk from the 1 in 1000 year event (Flood Zone 2) then the Exception Test would not have been required and gaining approval for the proposed residential sites would have been relatively straightforward following completion of the Sequential Test.

Figure 12 – Draft revised Flood Zones

Legend

Development Allocations Flood Zone 3 revised Flood Zone 2 revised

OS Licence: © Crown Copyright 100023297 (2010)

2010s4000 DBC Level 2 SFRA V3.0.doc 17

3.2 Development Implications and Issues for Existing Risk 3.2.1 Flood Flow Pathways The River Skerne has been modelled from the railway bridge (downstream of Albert Road) to Parkside (just downstream of South Park). The modelling shows that for the 1 in 100 year event, the River Skerne first starts to come out of bank near to the junction of Parkside and Grange Road (downstream of South Park). Shortly afterwards, the Skerne starts overtopping opposite South Park, towards the school. This area then begins to fill towards Marlborough Court (see the bottom part of Figure 13a).

Figure 13 – a) Main flood flow pathway early Figure 13 – b) Main flood flow pathway later stages stages

Flood Depth (m)

OS Licence: © Crown Copyright 100023297 (2010)

Following this, the Skerne comes out of bank immediately downstream of Chesnut Street (left bank) and begins to flow down Valley Street North (and the properties adjacent to the Skerne, this is within TCF site F). This flow pathway continues to the A167 – B6270 roundabout. At this point the Skerne overtops immediately downstream of John Street (left and right bank). The floodwater flows down Valley Street North and joins with the Chesnut Street flow path (within TCF sites E and F). This combined flow pathway can be seen in the top part of Figure 13a. On the right bank at John Street, the floodwater flows round the defences and back upstream into the area between Cocker Beck and the railway bridge (part of TCF site E). The River Skerne in this area is confined by old brick buildings which have been built directly against the river bank over the years. The flooding and flood pathways in this area are therefore between buildings. The model assumes that there is some flow through the buildings (via gaps in doors, vents etc) but this area should not be classed functional floodplain due to the restriction of flow by the existing buildings. While flooding in Darlington urban area is taking place, the area downstream around South Park also floods to a greater extent. The cricket ground area fills and the Skerne

2010s4000 DBC Level 2 SFRA V3.0.doc 18

overtops at South Park, eventually filling this entire area to depths of around 1m (see the bottom part of Figure 13b). The final part of the flooding event sees the John Street/Cocker Beck flood flow pathway extending all the way down to TCF site B. This is an important issue as TCF site B does not flood directly from the Skerne but via a flow pathway upstream. If this flow pathway were blocked, then TCF site B would not flood. The top part of Figure 13b shows the combined flow pathway reaching TCF site B as it starts to fill this area. 3.2.2 Flood Zones and the Functional Floodplain The revised Flood Zones (Figure 12) show that around two thirds of TCF sites F and E (including the extension north of John Street) is within draft Flood Zone 3 with a further section in Flood Zone 2. The majority of TCF site B is in draft Flood Zone 3, around two thirds of TCF site A is within draft Flood Zone 2. Figure 14 shows the existing risk (with defences included) flood extents and includes the 1 in 25 year flood extent (draft Flood Zone 3b). This is quite a narrow extent and this flood event mainly uses roads as a flow pathway, between buildings. This event affects the areas adjacent to the River Skerne in TCF sites E and F. In this area, buildings have been built right up the banks of the River Skerne. Although the model has allowed for some flow through the buildings, this area would not normally be defined as ‘functional floodplain’ due to the existing development next to the river bank that would prevent flooding in this area to a degree. However, the areas at risk from the 1 in 25 year event further downstream (at the cricket ground) should be defined as functional floodplain, as these areas are generally undeveloped. 3.2.3 Loss of Floodplain Issues If residential development is proposed in Flood Zone 3 then the Exception Test needs to be passed (following completion of the Sequential Test). Part of the Exception Test is to ensure that the development does not increase flood risk to others. One way in which development can increase flood risk to others is by developing in the fluvial floodplain. Building in the floodplain can displace floodwater and increase levels upstream and downstream. The Environment Agency state that if development does need to go ahead in the 1 in 100 year fluvial floodplain, then there should be no loss of flood flow or flood storage capacity as a result of the development. As can be seen in Figure 14, large parts of TCF sites E, F and B are within the 1 in 100 year flood extent. Although this area is already developed, the development is not intensive, with open areas used for parking and storage. One of the policy aims within PPS25 is to reduce the overall level of flood risk in the area through the layout and form of the development. This can be achieved by increasing conveyance (flow) through the site or by providing compensatory flood storage either on site or upstream of the development. Initial model runs were undertaken to see if developing in the areas in Figure 14 would have a significant increase in flood levels downstream. This showed that blocking flow from entering the development areas would increase levels by a small degree immediately upstream and downstream at South Park. However, when the river floodplain was widened in the model to take more flow, there was no increase in levels upstream and a negligible increase downstream at South Park. This indicates that a reduction in conveyance (flood flow) is of more importance than a reduction of flood storage capacity in this area. This means that adjusting the development layout and increasing conveyance should be able to mitigate any increase in flood risk and could provide an overall net benefit to Darlington. More details on the impact in development in flooding can be found in 1.2.6.

2010s4000 DBC Level 2 SFRA V3.0.doc 19

Figure 14 – Existing Risk Flood Extents

OS Licence: © Crown Copyright 100023297 (2010)

3.2.4 Flood Depths and Hazard Figure 15 shows the modelled flood depths for the 1 in 100, 1 in 100 + climate change (+cc) and 1 in 1000 year flood events. Flood depth is important as it shows the level of flood risk to people and whether mitigation options will be viable. Environment Agency guidance is that new development should be flood free during the 1 in 100 year event, flood risk mitigation measures should also take into account climate change (i.e. it should be added to freeboard). New development should be able to manage the risk from the 1 in 1000 year event. This does not necessarily mean that the development should be flood free for the 1 in 1000 year event, but measures such as flood resilience (see glossary) should be in place up to this event. Table 1 below (taken from Volume I of Darlington BC’s Level 1 SFRA) shows the typical depths where certain mitigation measures would be applicable.

2010s4000 DBC Level 2 SFRA V3.0.doc 20

Table 1 - Suggested Screening Criteria for Mitigation Measures Depth of Inundation* Comments 0 to 1.0 m Sustainable mitigation and flood risk management may be feasible for both housing and employment purposes. There is a greater likelihood that the Exception Test can be passed. 1.0 to 1.5 m Mitigation is likely to be costly and may not be economically justifiable for low value land uses. Housing allocations are considered appropriate, provided flood risk can be managed or mitigated (e.g. by using lower levels for car parks or public areas). Floor level raising for employment purposes is unlikely to be economically viable and employment allocations should be reconsidered in favour of alternative lower risk sites. The likelihood of passing the Exception Test is lower. Above 1.5 m Flood risk mitigation measures are unlikely to be economically justifiable and both housing and employment allocations should be reconsidered in favour of alternative lower risk sites. Development is unlikely to be sustainable and the likelihood of passing the Exception Test is low. Notes: * Based on predicted depth of inundation for the 1% (Fluvial) event + 20% additional flow for Climate Change as per PPS25. Environment Agency flood zone data

In general, the flood depths are low for the TCF sites. For the 1 in 100 year +cc event, depths rarely get above 0.5m. This means that mitigation measures should be possible for residential and employment allocations. However, this does not take into account whether floodplain compensation can be found. For the 1 in 1000 year event flood depths are more consistently above 1m for TCF site B. TCF site F is also at risk of flood depths of around 1m. Elsewhere, flood depths are between 0.5 and 1m. Although flood depths are greater for the 1 in 1000 year event, they are not excessive and it should be possible to design flood resilience and resistant measures to mitigate this residual risk. Figure 16 shows the flood hazard (see glossary) grid for the TCF sites. These results are described after the figures.

2010s4000 DBC Level 2 SFRA V3.0.doc 21

Figure 15 – Existing Risk Flood Depths

1 in 100 Year Flood Depth 1 in 100 Year plus Climate Change Flood Depth 1 in 1000 Year Flood Depth

OS Licence: © Crown Copyright 100023297 (2010). OS Licence: © Crown Copyright 100023297 (2010). OS Licence: © Crown Copyright 100023297 (2010).

2010s4000 DBC Level 2 SFRA V3.0.doc 22

Figure 16 – Existing Risk Flood Depths

1 in 100 Year Flood Hazards 1 in 100 Year plus Climate Change Flood Hazards 1 in 1000 Year Flood Hazards

OS Licence: © Crown Copyright 100023297 (2010). OS Licence: © Crown Copyright 100023297 (2010). OS Licence: © Crown Copyright 100023297 (2010).

2010s4000 DBC Level 2 SFRA V3.0.doc 23

Table 2 below (taken from Volume I of Darlington BC’s Level 1 SFRA) shows the flood hazard thresholds and aligns with the colours in Figure 16. As can be seen in Figure 16, flood hazard is generally low for the 1 in 100 and 1 in 100+cc year events (between caution and danger for some people). Table 2 - Flood Hazard Thresholds Flood Hazard d(v+0.5)+DF Description Alternative Name / Hazard Class 0 Safe (dry) None 0 to 0.75 Caution Low 0.75 to 1.5 Dangerous for some Moderate 1.5 to 2.5 Dangerous for most Significant Over 2.5 Dangerous for all Extreme

As development needs to be dry for the 1 in 100 year event (taking into account climate change), flood hazard is normally used as an indicator for the level of residual risk (the 1 in 1000 year event). The development does not have to be dry during this event, but it should be safe. For the 1 in 1000 year event, the flood hazard is currently classed as ‘danger for most people’ i.e. significant risk for large parts of the TCF sites. However, this does not take into account any mitigation measures which may (for example) raise the land, reducing the flood hazard. This is likely for TCF sites E and F, but for site B, the flood flow pathway is not directly from the Skerne but from further upstream. If this flow pathway is blocked, site B will only be at risk from the 1 in 1000 year event, leaving it exposed to ‘significant’ risk. 3.2.5 Access and Egress The Environment Agency stipulates that new development in the floodplain should have safe access and egress for the 1 in 100+cc event. The extent of this flood event can be seen in Figure 14 and the flood depths for this event can be seen in Figure 15. The flood depths for the climate change event are generally between 0 and 0.5m (with a few places above 0.5m). Raising access and egress roads above this event should not be too difficult due to the shallow depths involved. For development to go ahead in site B, it is likely that following mitigation, the flood flow pathway to this site (from upstream in the River Skerne) would be blocked. This would still leave the site at risk from the 1 in 1000 year event. It is not necessary to raise access roads above this flood event. However, if this flood event did occur, flood depths of between 1 and 2m would occur potentially hindering access and egress. For sites E and F, land raising above the 1 in 100 year+cc level is likely. It will therefore be relatively straightforward to raise access roads (and a network of linked drives) to this level. This would provide dry access and egress up to the climate change event. The flood depth of the 1 in 1000 year even would also be reduced by land raising. 3.2.6 Mitigation Options Considered Within the model, different flood risk mitigation scenarios have been modelled to see if development will be safe once built and will not increase flood risk elsewhere. Due to the low flood depths predicted, mitigation measures which would remove flooding from development during the 1 in 100 year+cc event should not be too problematic. However, what is important is the impact these mitigation measures have downstream. It is Environment Agency policy that there should be no increase in flood risk upstream or downstream due to a loss of floodplain. In addition, one of the Flood Zone 3 policy aims within PPS25 is to reduce the overall level of flood risk in the area through the layout and form of the development. As mentioned earlier (in 1.2.3), the modelling results indicate that loss of flood conveyance rather than loss of floodplain storage is of more importance when it comes to reducing the impact on flood levels upstream or downstream. This section summarises the mitigation options considered and concludes with the most preferable option when balancing the development needs and flood risk.

2010s4000 DBC Level 2 SFRA V3.0.doc 24

3.2.7 Raised Development One of the mitigation scenarios modelled is to raise TCF sites E and F above the 1 in 100 year+cc flood level. This removes flooding from sites E and F and blocks the flow pathway to site B. Roads would also be above the 1 in 100 year+cc level providing safe access and egress. Flood resilience measures would need to be designed in up to the 1 in 1000 year event (approximately an additional 0.5m in height). This scenario generally shows a 13mm increase in levels upstream of TCF site E and downstream, at South Park, an increase of between 11mm and 31mm. Upstream the Skerne is within steep sides, so this increase is not passed on to any other development, the impact diminishes further upstream. The downstream increase in levels is not passed on to the downstream development within central Darlington as the River Skerne still stays in bank. The increase in levels is passed on to South Park and the adjacent school playing field/cricket ground area. The result is a slight increase in flood depths here. Figure 17 shows the existing flood risk (blue) and the flood outline if the development was raised (purple outline). This shows the TCF sites E, F and B no longer at risk. There is a minor increase in flood extent in some isolated locations in South Park and upstream at the railway bridge (although this impact is too small to be seen within Figure 17). The main impact that can be seen is a slight increase in depths at South Park. In summary, this mitigation scenario allows safe development but there is a slight increase in flood levels upstream and downstream. This increase in levels does not increase flood risk to any development but it is a standard Environment Agency response to object to any loss of floodplain. This means that an increase in flood conveyance or some form of compensatory flood storage may need to be found on site or off site to compensate for the loss of floodplain. However, as the increase in levels is minor and no existing development is put at increased risk, this may not be an essential requirement. The Environment Agency will confirm this following consultation on this report. 3.2.8 Flood Defences Flood defences on the left and right bank of the River Skerne within TCF sites F and J, up to the 1 in 100 year+cc event, would remove flooding to sites F and J and block the flow path to site B. This would have the same impact upstream and downstream as the raised development scenario (see Figure 17). There may also be some additional disadvantages which may preclude this option. The new development behind the defences will not be raised, so if the 1 in 1000 year event did occur, then the site would be at risk of flooding of depths of up to 2m and with high flood hazard ratings. In addition, a flood defence presents an additional hazard due to the risk of it failing. A breach in a flood defence produces high velocity, rapid inundation flood flows. There may also be an aesthetic constraint to flood defences. Riverside development is attractive due to the views. A raised defence would restrict this view to a degree although flood defence could be carefully integrated into the design to reduce this impact.

2010s4000 DBC Level 2 SFRA V3.0.doc 25

Figure 17 – Comparison of the raised TCF sites (or flood defences) against the existing risk for the 1 in 100 year+cc event

OS Licence: © Crown Copyright 100023297 (2010)

2010s4000 DBC Level 2 SFRA V3.0.doc 26

3.2.9 On Site Flood Storage This section involves the consideration of compensatory flood storage. If the 1 in 100 year area is raised and developed, then the loss of floodplain may need to be compensated (unless the Environment Agency confirm that compensation is not required). Two variations of compensatory floodplain storage can be considered here. Firstly, storage could be designed on site, either as underground flood storage (e.g. underground car park) or a ground level undeveloped amenity area. Alternatively, an undeveloped area upstream could be utilised for flood storage. Upstream storage is described under the next heading. There are some practical issues associated with on site storage, as like for like compensation would need to be found. This means that for any development within the floodplain, a similar area would need to be allocated for storage (free from development). This would reduce the housing numbers and not create any overall benefit in this regard. The other issue is related to subsurface storage. If the habitable floor level is built above the 1 in 100 year +cc level, the space beneath could be retained for flood storage during the 1 in 100 year +cc flood. This area could be used as a car park, with emergency access and egress procedures for the car park triggered by a flood warning system. However, the type of development that is likely to be able to handle this type of residential development is three storey town houses or (more practical) flats. This indicates that allowing the ground floor to flood may be a constrained option and other land uses or mitigation options should be considered. The situation is not as problematic with less vulnerable land use (offices, retail and other employment land use). The Exception Test does not apply, but any development in the 1 in 100 year area would still need to be compensated. Ground floor flooding can be achieved more easily with less vulnerable land use e.g. offices on stilts (and car parking under this area) or low grade ground floor use with stock moved to higher levels and flood resilience measures on the ground floor. It is also much easier to evacuate this type of development following a flood warning. An example of residential flats with the ground floor being used as a car park (which would flood in an extreme event) can be seen below.

3.2.10 Raised Development and Upstream Compensatory Storage If onsite storage looks unfeasible, then upstream flood storage could be considered. Upstream storage requires an area of undeveloped land that is not at risk from the 1 in 100 year flood (but close to it) to be retained as flood storage. This is normally an area within the 1 in 1000 year flood extent. This area can be excavated so that it floods during the 1 in 100

2010s4000 DBC Level 2 SFRA V3.0.doc 27

year event. This area needs to be large enough to contain the volume of flood water that will be displaced. The ‘Storage Requirement’ heading in Table 3 below shows the volume of floodplain that is taken up by the proposed development during the 1 in 100 year event. This is translated into the storage areas required depending on what depths are used. For example, an area of 5 hectares would be required for a 0.76m deep compensatory flood storage basin. Table 3 – Compensatory flood storage volumes Storage Requirement (m³) 38,000 Available Area (hectares) 2 5 10 15 20 Required Depth (m) 1.90 0.76 0.38 0.25 0.19

Depending on the depth of the storage location, an area of between 2 and 20 hectares would be required immediately upstream of site E to compensate for the loss of floodplain. The further upstream the storage location, the larger the area of compensation required. This principle was tested within the hydraulic model. If the green area between Haughton Road and the River Skerne was used for upstream storage, a volume of around 42,000 m3 would be required. This is greater than what would be required immediately upstream of the development sites. The model showed that storing this volume at the site mentioned does bring the increase in levels from raising TCF sites, back to around the existing levels. In order to confirm these findings, this would need to be modelled in more detail. Any flood storage area will need to be controlled so that it will only flood during low probability events via an inlet. The flood water will naturally flow out via a designed outlet. As a large undeveloped area is required, finding this land and obtaining it will be difficult. There are few areas immediately upstream and the further upstream you go, the more land is required. In addition, there is a river restoration scheme upstream of the main railway line in Darlington. There will be resistance to any change in flows on the Skerne as this will affect this restoration scheme (although it may be possible to only influence flows during extreme flood events). The modelling has also been used to assess the amount of storage that would be required to stop the River Skerne overtopping in the centre of Darlington during the 1 in 100 year flood event. This would effectively remove flood risk from the Skerne to Darlington. If this were a viable option, it may attract part funding form the Environment Agency (although it would still have to be high enough of a priority to justify the amount of funding). However, the storage volume required is around 685,000m3 and far in excess of the displaced volume described earlier. As finding a location for the displaced volume is unlikely, finding an area for this greater volume will be very unlikely and it is recommended that this should not be considered any further at this time. 3.2.11 Amenity River Corridor and Raised Development As there are still some feasibility questions surrounding the above options, a final option was considered. This option involves designing a widened floodplain corridor either side of the River Skerne. This would increase flood conveyance and create additional storage in order to reduce the downstream and upstream increase in flood levels as a result of raising the developments. This corridor would also be designed as an amenity space to improve pedestrian access to the river, including green areas and cycle paths etc. This option has been considered as the modelling results show that reducing flood conveyance (flow) has more of an impact on flood levels than reducing floodplain storage on the Skerne. Increasing conveyance by creating a widened floodplain corridor and/or changing the development layout should be able to reduce any downstream or upstream impact of developing in the floodplain. This widened floodplain area would need to be lowered to the 1 in 25 year flood level (for example). This area would actually need to be over deepened and then recapped to mitigate against any land contamination. When a 1 in 25 year flood occurs, access to this area would

2010s4000 DBC Level 2 SFRA V3.0.doc 28

be restricted (following a flood warning) and the area would be allowed to flood. The topography would be designed so that all of the 1 in 100 year+cc flood flow would be directed into this channel (through bunds and lowering ground levels). The 1 in 1000 year flood would be allowed to come out of this channel and flow through open spaces in the development, but to a low depth. Irrespective of this, flood resilience measures should be designed to the 1 in 1000 year flood level for this new, raised development. A 30m wide floodplain corridor was modelled, with bunds directing the 1 in 100 year+cc flow into this corridor. The results from modelling this scenario show that there is very little impact downstream and no impact upstream compared to the existing risk. There is no impact upstream as the increase in conveyance from having a wider flood channel allows more of the peak flow to move downstream than a narrow channel with raised development. This option still restricts the natural flood flows to a degree and raises levels slightly (more details on the downstream impact are below Figure 18). Figure 18 shows a comparison of the existing risk (blue) and the flood outline if a floodplain corridor were constructed (green outline). This shows the floodwater directed away from TCF sites and into the channel.

2010s4000 DBC Level 2 SFRA V3.0.doc 29

Figure 18 – Comparison of the 30m floodplain corridor against the existing risk for the 1 in 100 year+cc event

OS Licence: © Crown Copyright 100023297 (2010)

Downstream Impact As a result of directing the peak flow of the 1 in 100 year+cc event into the amenity corridor, river levels downstream (at the South Park area) generally increase by 2mm. This small downstream impact can be put down to the small increase in volume that is conveyed downstream. The flood depths in the TCF area are low (between 0 and 0.5m), this means the TCF flood volume that needs to be mitigated is low. Creating a widened corridor, flattens the flood hydrograph, giving a lower peak flow compared to a normal channel width and raised development. This increase in river levels shows a minor increase in flood extent in several peripheral areas around South Park (Figure 19 shows the only areas where an increase in extent is seen). The

2010s4000 DBC Level 2 SFRA V3.0.doc 30

increase in extent does not put any additional properties at risk of flooding as it predominantly occurs in parkland. No emergency access and egress routes are affected by this minor increase in extents. A comparison of the flood depths within the flood extent at South Park for the 1 in 100 year+cc event has also been made. A number of locations were selected within the depth grids of the two events. On average, an increase in flood depth of 8mm was seen in extent shown in Figure 19 as a result of the minor increase in river levels. This mitigation option therefore appears to allow development to go ahead (with some development restrictions due to the floodplain corridor) with only a minor increase in levels downstream, that does not put any additional properties at risk or restrict emergency access and egress.

Figure 19 – The 1 in 100 year+cc existing risk event (blue) and the increase in extent from the corridor option (red)

OS Licence: © Crown Copyright 100023297 (2010)

3.2.12 Development Sequence Issues This Level 2 SFRA looks at flood risk to a proposed regeneration area and how it can be managed on a strategic level. I.e. it can be seen that there are flood risk issues through the centre of Darlington and strategic solutions (see above) can be proposed. However, strategic solutions may need contingency plans when different sites are brought forward at different times. This may be an issue for the TCF sites. It is anticipated that sites A, B and H will be

2010s4000 DBC Level 2 SFRA V3.0.doc 31

brought forward in the next 5 years, while sites E and F may take much longer to come forward. A summary table outlining mitigation requirements and how this impacts on each site is included in section 3.3. Some details on how to bring site B forward in advance of sites E and F are provided below. Indications are that site B may come forward for development first. Flooding to this site does not come directly from the River Skerne but from a flood pathway that originates near Chesnut Street (see Figure 13). The proposed mitigation strategy includes raising parts of sites E and F and/or designing a flood corridor, directing the flood flow through this corridor rather than the development, cutting off the flood pathway to site B. If site B is developed before these mitigation options are implemented, it will remain exposed to flood risk. Options are therefore required to manage the risk to site B in the short to medium term (before the flood route is cut off during the regeneration of the land adjacent to the Skerne). These are outlined below.

Flood Resilience To manage the risk on site B, one option could be to design flood resilience measures up to the 1 in 100 year+cc event. Although the normal requirements are for the development to be flood free for the 1 in 100 year event, this could be seen as a temporary measure as in the medium term, development at sites E and F will stop the flood pathway to B. In addition, if mixed use development is built here, the land use vulnerability would be downgraded (compared to emergency services), which may be an acceptable short term compromise (if constructed with flood resilience measures to the 1 in 100+cc event).

Raised Development Alternatively, site B could be raised above the 1 in 100 year+cc level. This was modelled and it was shown to locally increase the flood extent due to pooling against the raised development, but this increase does not put any additional properties at risk (just roads). In the immediate vicinity of the raised development, flood depths are modelled to increase by around 200mm due to pooling against this raised development. However, the topography around the new development could easily be designed to direct this pooling of flood water away from site B back into the River Skerne. Figure 20 shows the difference in flood extents with site B raised. The flood extent does not increase further downstream in South Park but depths increase marginally by 7mm for the 1 in 100 year event and 14mm for the climate change event. Existing development is not put at risk due to this small increase in flood depth.

2010s4000 DBC Level 2 SFRA V3.0.doc 32

Figure 20 – Comparison of raising TCF site B against the existing risk for the 1 in 100 year+cc event

OS Licence: © Crown Copyright 100023297 (2010)

Raised Defences The final option available to mitigate flood risk to Site B (before sites E and F are developed, blocking the flood flow route) are raised defences. These defences could be constructed as a temporary measure for the short/medium term before sites E and F are developed. The flow pathway from the Skerne between Chesnut Street and Russell Street could be defended. This would remove flood risk to Site B but there could be an increase in flood risk downstream as a result. Alternatively, a series of bunds could be constructed adjacent to the A167, diverting the flood water away from site B, back into the Skerne via the A167 (before the roundabout). The floodwater could be directed across the A167 by constructing several speed bumps perpendicular to the road. This would have the advantage of passing on the small increase in flood levels to South Park, rather than to the existing development further upstream.

2010s4000 DBC Level 2 SFRA V3.0.doc 33

Modelling results have shown that preventing Site B from flooding and passing this flow downstream increases flood depths in South Park by 7mm for the 1 in 100 year event and 14mm for the climate change event. This bund system could be extended to protect site A, a small part of which is at risk of flooding from the 1 in 100 and a further extent at risk from the 1 in 1000 year event. Due to the shallow flood depths, the bunds would only need to be around 500mm high (including freeboard) to remove flooding to the site from the 1 in 100 year+cc event. Once sites E and F are developed with the amenity corridor, the bunds could be removed. Figure 21 shows the potential line of the bunds and where this flood flow would be directed back into the Skerne. The bund location is in yellow, the 1in 100 year+cc flood extent is in blue and the grey outlines are the proposed development sites. The River Skerne centre line is the blue line, the dashed blue line is the flood pathway with the bunds in place.

Figure 21 – Potential bund location to remove short term flood risk to Site B

OS Licence: © Crown Copyright 100023297 (2010)

3.3 Mitigation Strategy The TCF sites comprise a large redevelopment area which has a number of different land owners. The development of the TCF will take place in stages, not at the same time. This SFRA has described flood risk and the potential mitigation measures in a way that assumes all of the TCF is redeveloped at the same time. It is therefore important to identify the key messages for the individual parts of the TCF before the next stages of planning (i.e. master planning) goes ahead.

2010s4000 DBC Level 2 SFRA V3.0.doc 34

Table 4 separates out each site and shows how each site fits into the overall proposed flood risk management strategy. This includes the flood risk management requirements for each individual site. This will ensure that as individual sites are developed, they do not increase risk to others, but form part of the overall flood risk management solution for this area. 3.3.1 Steps to Development There are a number of flood risk mitigation steps that need to be taken up to the development of each of the TCF sites. All of these steps should be completed in conjunction with the Environment Agency. Table 4 provides specific details for each site. 1. The River Skerne 2D hydraulic model (hydrology and additional survey - see 3.5 paragraph 4) should be updated before mitigation measures are proposed in the masterplan. This stage should confirm the limited downstream impact of the widened flood corridor. The outputs should be reviewed by the Environment Agency. 2. Based on Table 4, undertake the strategic design of mitigation measures for each TCF site for the TCF Area Action Plan masterplanning stage. This stage should include the projected costings of the mitigation measures. The outputs should be reviewed and approved by the Environment Agency. 3. Floor raising and flood resilience levels should be based on the updated modelling outputs (once reviewed by the EA). 4. Developer flood risk assessment (FRA), based on the strategic mitigation measures outlined for the individual sites in the Area Action Plan / masterplan.

2010s4000 DBC Level 2 SFRA V3.0.doc 35

Table 4 – Mitigation strategy for the TCF sites Site Flood Risk Mitigation Requirements Impact on others and issues within the larger name mitigation strategy? and type A - Flood risk originates from In the short / medium term, the minor flood risk to this Blocking the flow route to this site could increase residentia overtopping on the left bank of site should be reduced by a raised defence on the flood risk to development downstream. The extension l / mixed the Skerne at Chesnut Street. Skerne (blocking the flood flow pathway) or an of the site B bunds is therefore the preferred option. This flood pathway flows down extension of the bunds proposed for site B. This is This would have a minor downstream impact at Valley Street North and required because site A is expected to come forwarded South Park (see 3.2.12). eventually reaches the site. A before the green corridor mitigation strategy in sites E small part of the site is at risk and F. This should be paid for by the developer with an Once the widened flood corridor has been from the 1 in 100 year event. agreement in place to maintain this asset until the completed, this site will be protected by this larger Around two thirds of the site is larger flood defence scheme is constructed. mitigation scheme. The bunds or raise defense on at risk from the 1 in 1000 year the Skerne could then be removed. event. Flood depths are In the long term the flood flow route to the site will be generally below 0.5m for the 1 removed by the upstream development (longer term in 1000 year event. development aspiration).

Flood resilience measures up to the 1 in 1000 year flood event may be an Environment Agency requirement. B - Flood risk comes from In the short / medium term, a raised defence between Blocking the flow route to this site could increase residentia overtopping on the left bank of Chesnut Street and Russell Street on the Skerne flood risk to development downstream. The on site l / mixed the Skerne at Chesnut Street. (blocking the flood flow pathway) or bund system at bunds are therefore the preferred option. The bunds This flood pathway flows down this site, directing flow into the Skerne should be could be extended to reduce the smaller flood risk Valley Street North and constructed. The flood defence or bund should be set extent to Site A. eventually reaches the site. at the 1 in 100 year +cc event. This should be paid for The entire site is at risk from by the developer with an agreement in place to Once the widened flood corridor has been the 1 in 100 year event. For maintain this asset until the larger flood defence completed, this site will be protected by this larger this event depths are below scheme is constructed. mitigation scheme. The bunds or raise defense on 0.5m, for the 1 in 1000 year the Skerne could then be removed. event, depths are over 1m. In the long term, the bunds can be removed as the flood risk to the site will be prevented due to the

2010s4000 DBC Level 2 SFRA V3.0.doc 36

Site Flood Risk Mitigation Requirements Impact on others and issues within the larger name mitigation strategy? and type upstream development.

Flood resilience measures up to the 1 in 1000 year flood event may be an Environment Agency requirement. E and F - The Skerne overtops at These sites are part of the widened floodplain corridor The development will need to be set back from the residentia Chesnut Street (left bank) and mitigation scheme. Land and floor raising should put river. Initial modeling estimates show that l / mixed John Street (both banks) these development sites above the 1 in 100 year+cc development should be set 30m back from the river. flooding site F east of the event. Flood resilience measures should be designed The river corridor area should be used for amenity Skerne. The flood pathway up to the 1 in 1000 year event. A 30m wide floodplain purposes (footpaths, cycle routes, green areas), with flows round the defences and corridor has been initially modeled. This will involve houses opening out on to this river frontage. The back upstream filling site E to widening an area either side of the river. river corridor should be developed before the the west of the Skerne. This development and should extend through the entire occurs during the 1 in 100 year The river corridor may form part of an overall flood length of the TCF site adjacent to the River Skerne. event. Further parts of the site defence scheme for the Skerne (subject to feasibility The entire corridor length may need to be completed are flooded during the 1 in studies). Funding for this scheme could therefore come at once, because if only one section is completed, as 1000 year event. Flood depths from the government (EA), DBC, developers and the channel narrows again, flood levels may increase are below 0.5m for the 1 in 100 raising local levees. downstream. Ideally, if one part of the site had to and generally between 0.5 and come first, the upstream part of the TCF sites E and 1m during the 1 in 1000 year F should be prioritised. event. Sites C, These sites are not at risk of flooding and can be developed independently of the other TCF sites. Flood risk assessments will be required as the G and H sites are over 1 hectare in size.

2010s4000 DBC Level 2 SFRA V3.0.doc 37

3.4 Next Stages In accordance with current planning guidance, the planning process ensures only appropriate development in areas vulnerable to flooding. This includes adopting a precautionary approach to decisions based on estimates of the present and future impact of flood risks. Whilst the Level 1 SFRA focuses on delivering a strategic assessment of flood risk within Darlington BC, this Level 2 SFRA has gone one step further in investigating flood risk in more detail at specific potential development allocations. This Level 2 SFRA has outlined which sites could be developed safely (and pass part c) of the Exception Test) and what mitigation measures will be required to do this. There is still a need for strategic mitigation measures to be designed and modelled at the masterplanning stage and for individual site specific flood risk assessments (FRA) to resolve detail and integrate the results into the master plan Area Action Plan and urban design. 3.4.1 Mitigation Strategies For the TCF sites, individual mitigation strategies are proposed, prior to the detailed planning application stage (i.e. before a FRA). This is required so the mitigation requirements can be linked in to the proposed development at an early stage. The outline to these mitigation strategies are provided in Table 4 and the sequence of tasks leading up to development are listed before this table. This process should be completed in partnership with Darlington BC, the Environment Agency and any developers. This will enable flood risk management to become an integral part of the future redevelopment adjacent to the River Skerne. The Environment Agency will be aware of what is being proposed, they will be able to review the findings and give advice leading up to development of the TCF. This will prevent delays in the planning process and will allow transparency so that objections can be avoided. It will be essential for the individual mitigation strategies to tie in with the other sites. If a site comes forward for development, the impact on the other sites should be taken into account. This Level 2 SFRA proposed a solution that impacts on and benefits a number of the sites at the same time so this will be unavoidable. Table 4 attempts to identify where potential conflicts are and how these can be resolved. The mitigation strategies could be completed in tandem with the TCF Area Action Plan. Some details on the proposed development may be required (e.g. development briefs) in order to complete this. Following the mitigation strategies, site specific FRAs will be required. General FRA guidance for developers has been supplied within the Darlington BC Level 1 SFRA, which must be referred to (see Chapter 3 of Volume III). Elements of the FRA guidance are outlined below: • Appropriate land use in flood risk areas • Undefended areas – flood risk mitigation • Defended areas • Overtopping • Breaching • Public Safety and rapid inundation • Feasibility of flood risk mitigation

3.4.2 Linking the SFRA with the Planning Process The LDF Core Strategy will be submitted to the Planning Inspectorate in November 2010 and is expected to be adopted by the Council in July 2011. At this time, the relevant policies in the Core Strategy will begin to be used to assess all planning applications and to provide informal advice at pre-application stage. Other LDF documents will continue to be prepared to provide a comprehensive, thorough approach to managing flood risk across Darlington. These include:

2010s4000 DBC Level 2 SFRA V3.0.doc 38

Accommodating Growth DPD A PPS25 Sequential Test will be prepared to accompany this site allocations document in order to determine whether the proposed development can be located in Flood Zone 1 in preference to higher flood risk zones (Flood Zone 2 and 3) as well as to the parts of a development site at lowest risk from flooding based upon the rate of inundation and flood hazard maps provided in the SFRA. Town Centre Fringe Masterplan/Town Centre Fringe AAP The Matsreplan covers the wider Town Centre Fringe area and is not a statutory planning document but will help inform the subsequent Town Centre Fringe AAP. The Council will prepare a technical note to reflect the principles of the PPS25 Sequential Test to accompany the Masterplan. A formal PPS25 Sequential Test will be prepared to accompany the AAP which is expected to be published for Issues and Options consultation in Autumn 2011. This will include all of the issues identified for the Accommodating Growth DPD above but will also set out how the flood risk management strategy will be developed. The AAP will also incorporate appropriate policies relating to the management of flood risk from fluvial, surface water and sewers in this part of the Borough. The masterplan will detail and examine the flood mitigation strategy for the area based on the SFRA and make recommendations for the Area Action Plan. This is important as a strategic view of the area is required particularly in regards to the phasing of development and flood mitigation options to reduce flooding in the area. For example, as noted within Table 4, if works to the River Skerne in the 'Site E' area was to take place, this could alleviate flooding for Site A and B. There is also a Core Strategy policy to set out a river corridor along the River Skerne which will be addressed within the Masterplan. Making Places DPD Setting out detailed development policies, this DPD will include a policy that provides guidance on flood risk assessment requirements. Design SPD The revised SPD to be adopted after the Core Strategy in 2011 will continue to promote the approach to mitigate flood risk through high quality design, incorporating surface water drainage where appropriate to greenfield run off conditions. Planning Obligations SPD The forthcoming SPD may incorporate guidance on seeking developer contributions to mitigate flood risk, particularly in the Town Centre Fringe area. Development Management As part of the implementation process, the Development Control and Projects teams will use the SFRA when assessing planning applications and providing pre-application advice. Where appropriate, they will direct developers to use the SFRA and its supporting maps which are available on the Council's website www.darlington.gov.uk/planningpolicy. A briefing note will be prepared for the Development Management Team to explain flooding issues in Darlington, the use of SFRA and its flood maps to help provide advice in relation to development sites particularly for the TCF. This will enable the DM Team to become more familiar with the information contained within the SFRA and build confidence to use the document for pre application enquiries and planning applications. As part of this process, the Council will continue to work with the Environment Agency to ensure that consistent advice is given at pre-application stage and in response to One Stop Shop applications to help raise awareness of flood risk at windfall sites as they come forward outside of the LDF process.

2010s4000 DBC Level 2 SFRA V3.0.doc 39

3.4.3 Emergency Planning Appropriate emergency planning must be incorporated in any FRAs. Emergency planning can be a crucial tool in reducing the residual risk to both people and to lesser degree property. Current flood response plans must be considered if development is going to place a greater number of people in areas of high risk to determine whether the actual risk can be managed or not. Section 3.2 identifies where emergency access routes should be identified for the Level 2 SFRA sites as part of emergency planning measures. PPS25 requires the LPA to make the final decision as to whether the Emergency Planning issues have been taken into account with their development plans. This specifically refers to emergency evacuation (access and egress). The approval of emergency planning procedures is not in the remit of the Environment Agency and specialist assistance may be required from Emergency Planners, Local Resilience Forums and the Emergency Services for approval.

3.5 Summary Overall, development and the mitigation measures within TCF sites E, F and B would significantly reduce flood risk to Darlington as a whole. The areas earmarked for development already have existing less vulnerable employment land use (sites E and F) and highly vulnerable (ambulance station and fire station) in site B. The development will remove flood risk from these areas leaving only a residual risk from the 1 in 1000 year flood (which can be mitigated through flood resilience measures). By developing in the 1 in 100 year floodplain, a small increase in flood levels can be seen but in undeveloped, confined areas (South Park). By designing a widened floodplain corridor for mitigation and amenity purposes, the increase in levels downstream is negligible. Any increase in downstream levels will not put any other properties at risk of flooding. Strategic modelling has shown that upstream compensatory flood storage would reduce any increase in levels as a result of developing in the floodplain. However, obtaining the land to implement this may be difficult. One of, or a combination of the above mitigation measures should allow the Exception Test to be passed so the development will be safe and flood risk will not increase elsewhere. There will also be an overall benefit to Darlington as the existing risk from the 1 in 100 year flood will be removed through the redevelopment. However, the practical viability of the proposed mitigation measures needs to be confirmed and further modelled during the master planning stage (mitigation strategy). Darlington BC should also ensure that the flood risk mitigation strategy is communicated across all relevant planning functions and integrated into the development masterplans. This should be completed in consultation with the Environment Agency at every stage. Before any of the mitigation options are taken further, and detailed master planning is undertaken, the hydraulic model will need to be modified in a number of areas. Firstly, the hydrology should be updated as new calculation methods have become available since the original hydrology was completed. Hydrology refers to the amount of flood flow entering the river system and new estimates may show an increase or decrease in these flood flows. A topographic survey of parts of the River Skerne will also be required. As some cross sections in the original model have been copied and there is limited detail on some of the structures in the river (e.g. weirs), providing more survey sections will give a more accurate representation of the river channel. Again, this may show an increased or reduced risk of flooding. Locations where additional topographic cross sections are required include: • Main river structures e.g. weirs and bridges • Section of river between Chesnut Street and Russell Street • Victoria embankment upstream of South Park • Bridge at South Park • Embankment at South Park

2010s4000 DBC Level 2 SFRA V3.0.doc 40

A technical modelling note has been produced for this study. More technical details on the modelling and the recommendations for model improvements can be found in this note (2010s4000-W_N001-1 - Darlington ISIS-TUFLOW model summary). This can be obtained from the Environment Agency or JBA Consulting.

2010s4000 DBC Level 2 SFRA V3.0.doc 41

4. Critical Drainage Areas

4.1 Introduction During the Level 1 SFRA, historical surface water flooding incident data was collected from a number of stakeholders. This included Darlington BC, the Environment Agency and County Durham and Darlington Fire Authority. This data was overlaid on top of the Environment Agency's Areas Susceptible to Surface Water Flooding Maps (ASSWF) and used to validate the zones. These are national surface water maps produced by the Environment Agency to show potential surface water flooding extents. The methods used are broad scale and should only be used for strategic and emergency planning purposes. Using this data 'candidate' Critical Drainage Areas (CDAs) were identified in the Level 1 SFRA. CDAs are those areas identified from historical flood events and/or modelled data as having a significant risk from surface water flooding or subject to potential large changes in runoff due to development. PPS25 Practice Guide states that SFRAs should provide the evidence and recommendations for LPAs to understand the need for a Surface Water Management Plan (SWMP) by identifying CDAs within their borough. The figure below, taken from PPS25 Practice Guide, shows how SFRAs link to SWMPs and then to overall spatial planning.

Candidate CDAs identified in the Level 1 SFRA include, Pierremont, the town centre area either side of the Skerne and Eastbourne. These areas have been investigated further within this Level 2 SFRA through. • Detailed surface water mapping • Consultation with Northumbrian Water

This has enabled the Level 2 SFRA to confirm the candidate CDAs as CDAs. When a SWMP is undertaken, it is these areas that should be focus of the assessment.

2010s4000 DBC Level 2 SFRA V3.0.doc 42

4.2 Detailed Surface Water Mapping As part of the confirmation of CDAs and to gain a better understanding of surface water flooding in Darlington, detailed surface water mapping has been completed. The 2D modelling software developed by JBA called JFLOW (or JRAIN) was used to route rainfall over an elevation model. This is the same base tool used for the Environment Agency's Areas Susceptible to Surface Water Flooding maps (see Section 3.6 of Volume II Level 1 SFRA) but with the following detailed and local improvements: • LIDAR data was used for the elevation model for the entire area. • The elevation model was modified via MasterMap data to include roads and buildings to help define flow paths. • The run-off from the surface was varied depending on whether an area was developed or green space, taking into account the variation of infiltration rates (water being absorbed by the ground). • An extreme 1 in 200 year rainfall event with a storm duration of 1 hour was chosen. • Three flood outlines were produced 1) less, 2) intermediate and 3) more susceptible to surface water flooding.

Most new sewers are designed to a 1 in 30 year design standard and hence sewer flooding problems will often be associated with more frequent storm events when a sewer becomes blocked or fails. In the larger events that are less frequent but have a higher consequence, surface water will exceed the sewer system and flow across the surface of the land. The capacity of the sewer network was therefore not taken into account in the surface water mapping. As a result, the surface water modelling, which is based on an extreme scenario, picks up overland flow paths that would be expected should the sewers surcharge. This is also the case for the more frequent storms when sewers could become blocked and surcharge at manholes, although flooding would be less extensive depending on the point in the sewer network where the blockage or failure has occurred. Considering both sewer and surface water flooding together is considered to be appropriate when taking a strategic view of flood risk in an extreme event from both these sources. A current and a future scenario were modelled. The future scenario takes into account the increased intensity of extreme rainfall predicted by climate change models and increased runoff from new developments on green space. The future scenario therefore provides a conservative and worst case scenario which is considered appropriate for a strategic study. This detailed surface water mapping has been used to confirm candidate CDAs (as CDAs) and larger surface water flooding locations, which were initially identified using the Areas Susceptible to Surface Water Flooding maps. Plans showing the detailed surface water mapping can be seen in Appendix A, Figures B1 to B5.

4.3 Critical Drainage Areas Section 2.5 in Volume II of the Level 1 SFRA provides an assessment of surface water flood risk. This includes historic flooding locations in the candidate CDAs of Pierremont, town centre area either side of the Skerne and Eastbourne. This part of the Level 2 SFRA builds on the initial Level 1 assessment with the findings from the detailed surface water mapping and further consultation. 4.3.1 Pierremont Description of the Issue In the Level 1 SFRA, the area around Pierremont was identified as a candidate CDA. This was because Pierremont was shown to have particularly extensive areas susceptible to

2010s4000 DBC Level 2 SFRA V3.0.doc 43

surface water flooding (according to the Areas Susceptible to Surface Water Flooding maps). Backing up of the sewers that flow into Cocker Beck is considered one of the flooding mechanisms in this area. In addition, this area has a number of historic flooding locations (unrelated to fluvial flooding). It was found that NWL is to undertake a scheme in the Pierremont area because during storms, rainfall entering the sewerage network has resulted in flooding to the properties on Pierremont Crescent. After the consultation meeting with NWL they confirmed that the design for a scheme to improve the hydraulic capacity of the system was completed in 2009. A new combined sewer outfall (CSO) and large sewer are to be installed with outfalls into Cocker Beck. The detailed surface water mapping in Figure 22 shows that there are areas of potential surface water flooding in Pierrmont. However, there are no clear flow pathways and the extents may be due to low lying areas that may be prone to ponding during heavy rainfall events. Surface Water Mapping

Figure 22– Detailed surface water mapping in Pierremont

OS Licence: © Crown Copyright 100023297 (2010)

Conclusion The detailed surface water mapping shows areas where surface water could pool during heavy rainfall events. There is a particularly extensive area at risk around Pierremont Crescent. Storm water can enter the combined system here causing it to discharge and flood

2010s4000 DBC Level 2 SFRA V3.0.doc 44

the surrounding area. NWL are undertaking a scheme to address these problems. As this is the only location within the Pierremont area that has a history of flooding, it has been concluded that this should NOT be confirmed as a CDA as a scheme is to be undertaken and there is no major development proposed in this area. 4.3.2 Town Centre Description of the Issue The Level 1 SFRA identified the town centre, either side of the River Skerne (near the fire station), as a candidate CDA. This was due to a cluster of historic flooding incidents and surface water flooding extents from the Areas Susceptible to Surface Water Flooding maps. Within this area, there are a number of outfalls into the Skerne. It is believed that there is an issue with these outfalls backing up. Backing up of the surface water system (that flows into the Skerne) could result in surface water flooding from drains and manholes, leading to ponding in low lying areas. The sewer system in this part of Darlington is old and prone to problems such as culvert collapse. It is often difficult to locate where the problems are in these older sewer systems. During the meeting with NWL, it was confirmed that there is an issue with the surface water sewer system here and that there is a history of sewer flooding in the area. NWL confirmed that there are issues in the Bedford Street area. NWL are considering a scheme for this area but at the time of publication not further details were available. Most of the sewers to the east of the Skerne pass under the Skerne via an inverted siphon. The dip in this inverted siphon is prone to sediment accumulation which may result in capacity issues during flood flows. A scheme is being considered for this area but it is not known whether it will go ahead or not. Figure 23 shows the detailed surface water flooding extents in this area. The roads that run north to south (parallel with the River Skerne) act as surface water flow pathways. Included in this is Bedford Street, which is a known problem location. There are also flow pathways and areas where surface water can pool, around the fire station and Park Place.

2010s4000 DBC Level 2 SFRA V3.0.doc 45

Surface Water Mapping

Figure 23– Detailed surface water mapping in the centre of Darlington

OS Licence: © Crown Copyright 100023297 (2010) Conclusion There is a history of surface water flooding here and NWL have confirmed that this is a known surface water problem location. Complications in this area include an old sewer infrastructure and an inverted siphon which takes some of the sewers under the River Skerne. The redevelopment of this area presents opportunities for a strategic drainage solution. Mixed use development is proposed for the fire station area (circled). Developer contributions to an NWL scheme may be possible here. Redevelopment could improve the surface water sewer arrangement and reduce runoff volumes. An upgrade or removal of the inverted siphon that passes under the Skerne may be one option. Increasing surface water discharges into the River Skerne should be resisted due to the existing fluvial flood risk on this river. This location has therefore been confirmed as a CDA. 4.3.3 Eastbourne Description of the Issue The Level 1 SFRA identified the Eastbourne area as a candidate CDA. This was due to the significant intermediate (and some high) surface water susceptible locations (from the Areas Susceptible to Surface Water Flooding maps) and a number of historic flooding incidents that tie in with these zones.

2010s4000 DBC Level 2 SFRA V3.0.doc 46

Figure 24 shows this candidate CDA and the detailed surface water mapping completed for this study. There is a potential surface water flow pathway flowing northwest to southeast. This spreads in Eastbourne Park which would provide surface water flood storage before continuing south-eastwards. The gardens to the north of Geneva Road also appear to show some surface water storage. From this point, the surface water flows along roads before pooling against the railway embankment south of Edgemoor Road. This is the only strong surface water flow route identified by the detailed surface water mapping; however, none of the historic surface water flooding locations align with this area. In addition, after meeting NWL in order to verify or reject this area as a CDA, they confirmed that they have no surface water sewer flooding issues in this area or any plans for a scheme. This would suggest that although there are some natural surface water flow pathways, the combined sewers and surface water (highway) sewers have enough capacity to cope with the flood flows. Surface Water Mapping

Figure 24– Detailed surface water mapping at Eastbourne

OS Licence: © Crown Copyright 100023297 (2010) Conclusion The detailed surface water mapping does not tie in with any historic flooding locations. In addition, NWL do not recognise this area as being vulnerable to surface water flooding. It is recommended that this area is NOT confirmed as a CDA. The playing fields at Eastbourne Park should be kept undeveloped as this area potentially provides surface water flood storage. Developing in this area could increase the strain on the surface water sewer system to the south.

2010s4000 DBC Level 2 SFRA V3.0.doc 47

4.4 Green Infrastructure Opportunities The Tees Valley Green Infrastructure Strategy promotes the integration of flood risk management measures with improving the quality of local community environments. It is seeking to develop a linked network of green corridors and green spaces by 2021 which provide multiple benefits throughout Teeside. Green Infrastructure (GI) is a strategically planned and delivered network of high quality green spaces and other environmental features. It should be designed and managed as a multifunctional resource capable of delivering a wide range of environmental and quality of life benefits for local communities. GI includes parks, open spaces, playing fields, woodlands, allotments and private gardens. GI should be provided as an integral part of all new development, alongside other infrastructure such as utilities and transport networks. GI can provide many social, economic and environmental benefits close to where people live and work including: • Places for outdoor relaxation and play • Space and habitat for wildlife with access to nature for people • Climate change adaptation - for example flood alleviation and cooling urban heat islands • Environmental education • Local food production - in allotments, gardens and through agriculture • Improved health and well-being – lowering stress levels and providing opportunities for exercise

The GI approach and flood risk planning is best approached spatially. There is a hierarchy of spatial scales in planning all land use, development and infrastructure. Broadly, these can be recognised as being undertaken at the sub regional scale, Borough, neighbourhood and site specific scale. This approach is supported by government policy, including CABE’s Grey to Green campaign. This campaign is calling for a shift in funding and skills from grey to GI. This means moving a proportion of investment in projects like road building and heavy engineering to networks of green spaces to provide flood protection and cut carbon emissions. PPS25 invites responsible parties to make ‘the most of the benefits of GI for flood storage, conveyance and sustainable urban drainage systems’. Planning Policy Statements and sub- regional Action Plans all provide policy support for planning investment in GI. A summary of the main priorities and actions from the Tees Valley Green Infrastructure Strategy for Darlington relevant to this SFRA are listed below. River Skerne Corridor • Identify opportunities for enhanced access and landscaping • Improve links with Central Park and Darlington town centre • Protect and enhance riverside habitats • Integrate with proposals in County Durham • Investigate opportunities for habitat creation throughout the urban sections of the corridor and into the countryside

River Skerne Corridor to West Park/Faverdale • Develop core path networks into surrounding communities • Investigate scope for additional tree planting and creation of wetland habitats • Incorporate green infrastructure elements within new development at West Park and Faverdale

2010s4000 DBC Level 2 SFRA V3.0.doc 48

4.4.1 Potential GI Opportunities Redevelopment and development of previously undeveloped land can provide GI opportunities. In already developed areas, watercourses are often culverted to create space for development. By opening up culverted watercourses, flood risk can be reduced, the amenity value of the area can be improved and biodiversity can be increased. Undeveloped greenfield areas often have natural surface water flow pathways or watercourses running through them. The surface water flow pathways can be identified by the detailed surface water mapping completed for this study. When planning new development, these natural pathways should be kept open and given space to flood. Again, when the development is complete, these corridors will allow the watercourses and surface water to flood naturally, create an amenity focus for the site and improve biodiversity. Locations where there may be GI opportunities in and around new developments, linking in the GI priorities/actions (listed above) are summarised in Table 5 below. Table 5 - Potential GI Opportunities in Local Plan (LP) Allocations’ Proposed Development Site GI opportunity EP2.6 Employment To the east of the site (the mainly undeveloped part) there are allocation at Lingfield several minor watercourses. One flowing north into the Skerne, the Industrial Estate other flowing to the south towards the River Tees. These should be kept open when this area is redeveloped for GI opportunities. EP8 Employment allocation The watercourse passing through the centre of the site should be north of Faverdale. kept open. An FRA should define the potential flood extent and therefore the green corridor width. Town Centre Fringe sites Proposed widened corridor through the Skerne to create space for flooding. The Skerne has been indentified as a strategic wildlife corridor and part of the strategic GI network, these actions (see list above) could be combined with this mitigation measure. EP2.2 Employment This site is adjacent to the Skerne (wildlife corridor and strategic GI allocation at Cleveland network - see the above list). Redevelopment should be kept back Trading Estate from the area at risk (a FRA should define this area). Opportunities to enhance the amenity value of the area along side the Skerne could be included when setting the new development back from the river. One option could be to open up and improve access to the river (e.g. linear pathway along side the Skerne) LP EP2.3 Employment Again this site is adjacent to the River Skerne (wildlife corridor and allocation at Albert Hill strategic GI network - see the above list). This part of the Skerne is the location of a river restoration project. Redevelopment should be kept back from the area at risk (during the completion on an FRA). Advantage could be made of the amenity value of the river restoration project here and providing improved access to the Skerne.

4.5 Future Studies 4.5.1 SWMP As described earlier, SFRAs should identify CDAs. SWMPs should then focus on these CDAs to develop a greater understanding of surface water flooding issues and identify potential solutions. Section 6.2.2 in Volume III of the Level 1 SFRA provides more details on SWMP requirements. In general, a SWMP will be required if there are complex flood mechanisms with a number of different stakeholders involved and for areas where significant new development and regeneration is proposed. The timing for the completion of the SWMP is dependant on available funding. Within Darlington BC, only one location has been highlighted as having complex surface water

2010s4000 DBC Level 2 SFRA V3.0.doc 49

flooding issues that may need a strategic solution. This CDA is the town centre site (see Figure 23). Surface water flood risk in this location aligns with the historic flooding data collected, the newly produced detailed surface water mapping and through consultation with NWL. The location also covers the proposed TCF Site B development site. Opportunities for improving the drainage system here during redevelopment are possible. A full SWMP may not therefore be required for Darlington BC. An individual study focused on the town centre site could make good use of any available funding that becomes available.

4.6 Surface Water Drainage and Development When major proposed developments come forward, opportunities for developing an Integrated Water or Drainage Management Strategy across development site boundaries should be explored, and a catchment led approach should be adopted. This approach has been recognised in the consultation paper by Defra, Making Space for Water. An integrated approach to controlling surface water drainage can lead to a more efficient and reliable surface water management system as it enables a wider variety of potential flood mitigation options to be used. In addition to controlling flood risk, integrated management of surface water has potential benefits, including improved water quality and a reduction of water demand through grey water recycling. Integrated drainage systems may be considered suitable for catchments where other development is being planned or constructed, and where on-site measures are set in isolation of the systems and processes downstream. Surface water drainage assessments are required where proposed development may be susceptible to flooding from surface water drainage systems. The potential impact upon areas downstream of the development, including the impact on a receiving watercourse, also needs careful consideration. The specific requirements for surface water drainage systems will need to be discussed with the Council’s Land Drainage Engineers, Environment Agency and Northumbrian Water. Consideration should be given to whether a “greenfield runoff approach” to the assessment of source control is appropriate. This method is generally satisfactory in the cases where the development is relatively small, isolated from other planned sites and the runoff processes are fully understood. The FRA should then conclude with an assessment of the scale of the impact, and the recommended approach to controlling surface water discharge from a proposed development. 4.6.1 SUDS This section provides a strategic summary of the applicability of SUDS techniques in Darlington. This is a broad scale assessment and should therefore not be used for assessing individual sites but it should be used for strategic planning. For more detailed assessments such as individual planning applications or site investigations, a comprehensive reporting service for specific locations can be found here: http://www.landis.org.uk/services/sitereporter.cfm Table 7 shows the soil types, the expected ground conditions from this soil type and the SUDS techniques that should be possible with these ground conditions. The SUDS techniques are categorised as storage (i.e. water stored on site and then slowly released) or infiltration (i.e. where surface water is allowed to infiltrate into the ground). Infiltration SUDS require ground conditions that allow the infiltration of surface water through the ground. Clay rich soils and areas with a high water table will not be suitable for infiltration SUDS. Table 6 shows the infiltration and storage SUDS techniques. For this broad assessment of soils, a simplified 1:250,000 soils dataset, derived from the more detailed National Soil Map. This is Cranfield University data and is available online. The drift geology data was obtained in GIS format from the British Geological Survey.

2010s4000 DBC Level 2 SFRA V3.0.doc 50

Table 6 - Suitability of SUDS Techniques SUDS Technique Infiltration Storage Green Roofs U V Permeable Paving V U Rainwater Harvesting U V Swales V V Detention Basins V V Ponds U V Wetlands U V Source: PPS25 Practice Guide

Table 7 - Strategic SUDS Applicability Area Soils and Drift Ground Conditions SUDS Implications Central and The drift is These soils absorb The soil type indicates that it south composed of till rainfall readily and should be suited to infiltration Darlington. overlain by freely allow it to drain SUDS systems. However, Including the draining loamy through to underlying particularly low lying areas next TCF sites soils layers. to the river may be subject to waterlogging when the water table is high. The remainder The drift is These soils are SUDS infiltration techniques of the Darlington composed of till slowly permeable may not be possible, only urban area and overlain by loamy and prone to winter SUDS storage techniques or peripheral and clayey soils waterlogging when underground storage basins locations with impeded the water table is may be applicable. drainage. high.

2010s4000 DBC Level 2 SFRA V3.0.doc 51

5. Conclusions and Next Stages

5.1 Town Centre Fringe 5.1.1 Conclusions This Level 2 SFRA has assessed flood risk to the key regeneration area called the Town Centre Fringe (TCF) by modelling the River Skerne and producing flood extents, flood depths and flood hazards. The aim of this modelling is to show whether the sites will be safe if developed and whether the development will increase flood risk downstream. A number of different flood mitigation measures have also been modelled. The results of the modelling and study show that the TCF sites are likely to pass the Exception Test if certain mitigation measures are used. A large part of the centre of Darlington adjacent to the Skerne is at risk of flooding. The area at risk includes business use but also some highly vulnerable sites (e.g. emergency services). Redeveloping the wider area would remove the risk of flooding to this site, and should the emergency services relocate, would reduce the risk to a highly vulnerable use. In order to remove the flood risk, a number of mitigation strategies have been considered. The mitigation measures will need to reduce flood risk to the new development, ensure flood risk does not increase to third parties downstream and if possible allow for some amenity benefit. The mitigation strategies modelled and considered include onsite and off site flood storage, flood defences, ground raising and a widened river corridor. The SFRA has taken into account the potential sequence of development. The lower sites (A and B) are likely to come forward in the next 5 years, the upper sites (E and F) are likely to come forward much later. The preferred mitigation option is to excavate a widened river corridor within Sites E and F to reduce peak flows and set the raised development back from the river. This would restrict flood flows to sites A and B, but temporary measures will be required here (flood bunds) as sites A and B will come forward first. By removing flooding from the floodplain, flood levels can increase downstream. The mitigation measures should be designed to pass this risk downstream to South Park. The widened river corridor means the increase in flood levels at South Park will be negligible and will not put any new development at risk. The redevelopment and flood mitigation strategy should be viewed as an integrated scheme for an overall reduction of flood risk in Darlington and the regeneration of a key part of Darlington. 5.1.2 Next Steps The masterplan and subsequent Area Action Plan should take the development of the TCF and flood mitigation strategy forward. The flood mitigation strategy should be integrated in the development design layouts. The following process should be followed when undertaking this next stage. • The River Skerne 2D hydraulic model (hydrology and additional survey - see 3.2.13) should be updated before mitigation measures are proposed in the masterplan. This stage should confirm the limited downstream impact of the widened flood corridor. The outputs should be reviewed by the Environment Agency. • Based on Table 4, undertake the strategic design of mitigation measures for each TCF site for the Masterplan and TCF Area Action Plan. This stage should include the projected costings of the mitigation measures. The outputs should be reviewed and approved by the Environment Agency. • Floor raising and flood resilience levels should be based on the updated modelling outputs (once reviewed by the EA). • Developer flood risk assessment (FRA), based on the strategic mitigation measures outlined for the individual sites in the Area Action Plan / masterplan.

2010s4000 DBC Level 2 SFRA V3.0.doc 52

5.2 Surface Water Management 5.2.1 Conclusions Detailed surface water mapping was undertaken for Darlington. In most places, this mapping does not convincingly tie in with any historic flooding locations and locations where Northumbrian Water (NWL) recognises significant surface water and sewer flooding problems. Only one location in the centre of Darlington near the River Skerne was identified. However, opportunities exist to manage surface water more effectively during the redevelopment stages, including the completion of Drainage Impact Assessments (DIAs). This may include assessing the applicability of SUDS (ground investigations) and setting runoff rates. Opportunities also exist to use existing and new green infrastructure to manage surface water runoff and provide amenity and biodiversity value. 5.2.2 Next Stages SFRAs should identify Critical Drainage Areas (CDAs) and Surface Water Management Plans (SWMPs) and then prioritise these CDAs and develop a greater understanding and solutions for the surface water flooding issues. In general, a SWMP will be required if there are complex flood mechanisms with a number of different stakeholders involved and for areas where significant new development and regeneration is proposed. The timing for the completion of the SWMP is dependant on available funding. Within Darlington BC, only one location has been highlighted as having complex surface water flooding issue that may need a strategic solution. This CDA is the town centre site (see Figure 23). Surface water flood risk in this location aligns with the historic flooding data collected, the newly produced detailed surface water mapping and through consultation with Northumbrian Water. The location also covers the proposed TCF site B development site. Opportunities for improving the drainage system here during the redevelopment is possible. A full SWMP may not therefore be required for Darlington BC. An individual study focused on the town centre site could make good use of any available funding that becomes available.

2010s4000 DBC Level 2 SFRA V3.0.doc 53

Appendices A. Figures

Provided separately

2010s4000 DBC Level 2 SFRA V3.0.doc I

B. Glossary of Terms

Attenuation Reduction of peak flow and increased duration of a flow event. Catchment Flood Management Plans (CFMP) A strategic planning tool through which the Environment Agency will seek to work with other key decision-makers within a river catchment to identify and agree policies for sustainable flood risk management. Climate change Long-term variations in global temperatures and weather patterns, both natural and as a result of human activity. Compensation storage A floodplain area introduced to compensate for the loss of storage as a result of land raising for development purposes. Design event A historic or notional flood event of a given annual flood probability, against which the suitability of a proposed development is assessed and mitigation measures, if any, are designed. Design flood level The maximum estimated water level during the design event. DG5 register Register held by water companies on the location of properties at risk of sewage related flooding problems Extreme Flood Outline Flood ‘zone’ maps released by the Environment Agency to depict anticipated 0.1% (1 in 1000 year) flood extents in a consistent manner throughout the UK Flood defence Flood defence infrastructure, such as flood walls and embankments, intended to protect an area against flooding to a specified standard of protection. Flood and coastal defence The Environment Agency, local authorities and Internal Drainage Boards Operating Authorities with legislative powers to undertake flood and coastal defence works. Flood Hazard Flood hazard = d (v+0.5) +DF Where: d is depth m v is velocity ms-1 DF is the debris factor with a value of 0-1

Floodmap A map produced by the Environment Agency providing an indication of the likelihood of flooding within all areas of England and Wales, assuming there are no flood defences. Only covers river and sea flooding.

2010s4000 DBC Level 2 SFRA V3.0.doc II

Floodplain Area of land that borders a watercourse, an estuary or the sea, over which water flows in time of flood, or would flow but for the presence of flood defences where they exist. Flood Estimation Handbook (FEH) Provides current methodologies for estimation of flood flows for the UK. Flood Risk Management (FRM) The introduction of mitigation measures (or options) to reduce the risk posed to property and life as a result of flooding. It is not just the application of physical flood defence measures. Flood risk management strategy A long-term approach setting out the objectives and options for managing flood risk, taking into account a broad range of technical, social, environmental and economic issues. Flood Risk Assessment (FRA) A study to assess the risk to an area or site from flooding, now and in the future, and to assess the impact that any changes or development on the site or area will have on flood risk to the site and elsewhere. It may also identify, particularly at more local levels, how to manage those changes to ensure that flood risk is not increased. PPS25 differentiates between regional, sub- regional/strategic and site- specific flood risk assessments. Flood risk management measure Any measure which reduces flood risk such as flood defences. Flood Zone A geographic area within which the flood risk is in a particular range, as defined within PPS25. Fluvial Flooding caused by overtopping of rivers or stream banks. Freeboard The difference between the flood defence level and the design flood level, which includes a safety margin for residual uncertainties. Greenfield land Land that has not been previously developed. ISIS ISIS is a software package used for 1-Dimensional river modelling. It is used as an analysis tool for flood risk mapping, flood forecasting and other aspects of flood risk management analysis. Local Development Framework (LDF) A non-statutory term used to describe a folder of documents which includes all the local planning authority’s Local Development Documents (LDDs). The local development framework will also comprise the statement of community involvement, the local development scheme and the annual monitoring report. Local Development Documents (LDD) All development plan documents which will form part of the statutory (LDDs) development plan, as well as supplementary planning documents which do not form part of the statutory development plan. Main River A watercourse designated on a statutory map of Main Rivers, maintained by Defra, on which the Environment Agency has permissive powers to construct and maintain flood defences.

2010s4000 DBC Level 2 SFRA V3.0.doc III

Major development A major development is: a) where the number of dwellings to be provided is ten or more, or the site area is 0.5 Ha or more or b) non-residential development, where the floorspace to be provided is 1,000 m2 or more, or the site area is 1 ha or more. NFCDD The Environment Agency's National Flood and Coastal Defence Database (NFCDD). Ordinary watercourse All rivers, streams, ditches, drains, cuts, dykes, sluices, sewers (other than public sewer) and passages through which water flows which do not form part of a Main River. Local authorities and, where relevant, Internal Drainage Boards have similar permissive powers on ordinary watercourses, as the Environment Agency has on Main Rivers. Permitted development rights Qualified rights to carry out certain limited forms of development without the need to make an application for planning permission, as granted under the terms of the Town and Country Planning (General Permitted Development) Order 1995. Planning Policy Statement (PPS) A statement of policy issued by central Government to replace Planning Policy Guidance notes. Previously-developed land Land which is or was occupied by a permanent structure, including the (often referred to brownfield land) curtilage of the developed land and any associated fixed surface infrastructure (PPS3 annex B) Ramsar Site Sites identified or meeting criteria set out in The RAMSAR Convention on Wetlands of International Importance. This definition has no legal status, but such sites are designated as SSSIs under the Wildlife and Countryside Act 1981 (as amended). Reservoir (large raised) A reservoir that holds at least 25,000 cubic metres of water above natural ground level, as defined by the Reservoirs Act, 1975. Residual risk The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented. Resilience Constructing the building in such a way that although flood water may enter the building, its impact is minimised, structural integrity is maintained and repair, drying & cleaning are facilitated. Resistance Constructing a building in such a way as to prevent flood water entering the building or damaging its fabric. This has the same meaning as flood proof.

Return period The long-term average period between events of a given magnitude which have the same annual exceedence probability of occurring.

2010s4000 DBC Level 2 SFRA V3.0.doc IV

Risk The threat to property and life as a result of flooding, expressed as a function of probability (that an event will occur) and consequence (as a result of the event occurring). Run-off The flow of water from an area caused by rainfall. Section 106 Agreement Section 106 of the Town and Country Planning Act 1990 (as amended) allowing local planning authorities to negotiate arrangements whereby the developer makes some undertaking if he/she obtains planning permission. These are known interchangeably as planning agreements, planning obligations or planning gain. Section 106 (Water Industry Act 1991) A key section of the Water Industry Act 1991, relating to the right of connection to a public sewer. Shoreline Management Plan (SMP) A plan providing a large-scale assessment of the risk to people and to the developed, historic and natural environment associated with coastal processes. It presents a policy framework to manage these risks in a sustainable manner. Site of Special Scientific Interest (SSSI) Sites notified under the Wildlife and Countryside Act 1981 (as amended) and the Countryside and Rights of Way (CRoW) Act 2000 for their flora, fauna, geological or physical features. Notification of a SSSI includes a list of operations that may be harmful to the special interest of the site. The Wildlife and Countryside Act 1981 (as amended) provides significantly enhanced protection for SSSIs. All cSACs, SPAs and Ramsar sites are designated as SSSIs. Standard of Protection (SOP) The design event or standard to which a building, asset or area is protected against flooding, generally expressed as an annual exceedence probability. Strategic Environmental Assessment (SEA) European Community Directive (2001/42/EC) on the assessment of the (SEA) Directive effects of certain plans and programmes on the environment. Strategic Flood Risk Assessment (SFRA) The assessment of flood risk on a catchment-wide basis for proposed development in a District. Sustainable Drainage Systems (SUDS) A sequence of management practices and control structures, often referred to as SUDS, designed to drain water in a more sustainable manner than some conventional techniques. Typically these are used to attenuate run-off from development sites. Sustainability Appraisal (SA) An integral part of the plan-making process which seeks to appraise the economic, social and environmental effects of a plan in order to inform decision-making that aligns with sustainable development principles. TUFLOW TUFLOW is a software package used for 2-Dimensional river modelling. It is used as an analysis tool for flood risk management analysis. Vulnerability Classes PPS25 provides a vulnerability classification to assess which uses of land maybe appropriate

2010s4000 DBC Level 2 SFRA V3.0.doc V

in each flood risk zone. Washland An area of the floodplain that is allowed to flood or is deliberately flooded by a river or stream for flood management purposes. Water Framework Directive (WFD) A European Community Directive (2000/60/EC) of the European Parliament and Council designed to integrate the way water bodies are managed across Europe. It requires all inland and coastal waters to reach “good status” by 2015 through a catchment-based system of River Basin Management Plans, incorporating a programme of measures to improve the status of all natural water bodies. Windfall sites Sites which become available for development unexpectedly and are therefore not included as allocated land in a planning authority’s development plan.

2010s4000 DBC Level 2 SFRA V3.0.doc VI

Offices at

Atherstone Doncaster Edinburgh Haywards Heath Limerick Newcastle upon Tyne Newport Northallerton Saltaire Skipton Tadcaster Wallingford Warrington

Registered Office South Barn Broughton Hall SKIPTON North Yorkshire BD23 3AE

t:+44(0)1756 799919 e:[email protected]

Jeremy Benn Associates Ltd Registered in England 3246693

Visit our website www.jbaconsulting.co.uk